Gnomes are the smallest humanoid species on the Discworld (a fictional flat world created by Terry Pratchett) ranging from four inches (10cm) to 2 feet (61cm) in height. There may be some confusion between the names gnome, goblin, and pictsie, which are more-or-less interchangeable terms for the same creature.
A gnome is a goblin living underground, while a goblin is a gnome coming up for air and a pictsie is a gnome fighting (see Nac Mac Feegle, although they're generally considered to be culturally different, and more recent books say they are a kind of fairy).
Rincewind and Twoflower meet a gnomeš in The Light Fantastic. Twoflower is disappointed, believing he should be dressed in brightly-coloured clothes and "more sort of... jolly". Rincewind explains: "He's six inches high and lives in a mushroom, of course he's a bloody gnome."
The gnome, whose name was given as Swires, is the same as Buggy Swires, a gnome corporal in the Ankh-Morpork City Watch.
Gnomes (or goblins) are able to make a good living as rat-catchers, because as well as being able to chase the rats into spaces where humans cannot, they can make several good meals, and indeed warm clothes, from a rat. Gnomes also tend to be surprisingly strong for their height.
Goblins are also featured in the short story Theatre of Cruelty, in which a children's entertainer forces them to perform as a Punch and Judy show. This story also reveals the existence of goblin dogs and alligators.
štechnically a goblin, as he was living above ground.